1. Field of the Invention
The present invention relates to an image forming apparatus, in particular, an image forming apparatus having a structure for pressing a developing unit against a photoreceptor unit.
2. Description of the Related Art
Japanese Laid-Open Patent Publication No. 2004-252255 (Document 1) describes an image forming apparatus having a structure for inserting a developing unit along a rail biased by a spring member, engaging therein, and positioning the rail at a reference position to allow a predetermined gap to be maintained between a photoreceptor drum and a developing roller.
Japanese Laid-Open Patent Publication No. 2006-337413 (Document 2) describes an image forming apparatus having a first frame for supporting a photoreceptor drum, and a second frame for supporting a developing roller. In the image forming apparatus, a cam is used to rotate the second frame between a location at which the developing roller makes contact with the photoreceptor drum and a location at which the developing roller is separated from the photoreceptor drum.
In each of the image forming apparatuses described in Documents 1, 2, accuracy in the gap is not necessarily sufficient between the photoreceptor drum and the developing roller due to the following reasons.
In the image forming apparatus described in Document 1, the developing unit is “displaced” on the rail, which changes the biasing force provided from the spring member, thus resulting in a less accurate gap between the photoreceptor drum and the developing roller.
In the image forming apparatus described in Document 2, the frame pushed by the cam is “displaced”, thus resulting in a less accurate gap between the photoreceptor drum and the developing roller.
The above-described unfixed gap between the developing roller and the photoreceptor drum can cause problems such as decreased image density and uneven image density.